The growing concern has also prompted Obama to suspend two days' worth of campaign events for the midterm congressional elections — a rare schedule change for a president who has in the past prided himself on a no-drama response to crises and more than once attended partisan fundraisers on the day of tragic events.

"Prime Minister Harper welcomed the president's effort to rally additional international support," said a readout of Thursday's chat, released by the PMO.

"He also indicated that Canada is a strong contributor to the international effort and that additional support would be announced in the coming days."

Texas health workers ordered to stay home

Dozens of health care workers who had contact with the man who died of Ebola in Dallas were asked Thursday to sign legal documents in which they agreed to stay home, as authorities ramped up efforts to limit the virus' spread.

The documents ask the 75 health care workers who had contact with Thomas Eric Duncan to agree not to go to public places or use mass transit, according to Judge Clay Jenkins, top administrator for Dallas County. The agreements are binding legal documents that can be enforced with a variety of remedies, Jenkins said, though he repeatedly declined to elaborate about specific punishments when asked by reporters and expressed confidence that everyone would comply.

"From 21 days after their last exposure, we are agreeing that they are not going to go on any form of public conveyance — any sort of public transportation," Jenkins said. "We are agreeing that they won't go where people congregate — public spaces — and we are agreeing that they will self-monitor and allow us to monitor them twice a day."

Do we need to worry about having an unsecure southern and northern border?- Marsha Blackburn, Tennessee Republican

It was one of several measures officials were taking Thursday amid an outbreak that has killed one person, infected two nurses and rattled nerves across the nation.

An official with the Centers for Disease Control and Prevention announced that the agency is expanding its Ebola investigation to include passengers on a Friday flight from Dallas to Cleveland that carried a nurse later diagnosed with the disease. Officials already had been contacting passengers on a flight that Amber Vinson, 29, took Monday on her way back to Dallas from a weekend trip visiting family.

Obama said on Thursday that he was considering appointing an Ebola "czar" to co-ordinate the fight against the virus in the United States. Obama also authorized calling up military reservists for the U.S. fight against Ebola in West Africa on Thursday.

Texas nurse Amber Vinson, left, steps from an ambulance at Emory University Hospital in Atlanta. Vinson, the second Texas nurse who had contracted Ebola, was flown to Emory Wednesday after being transferred from Texas Presbyterian Hospital. (Jerry Jordan/Reuters)

"It may be appropriate for me to appoint an additional person" to oversee efforts to contain Ebola, Obama told reporters after meeting aides involved in the fight against the disease.

Lawmakers have called for a czar and a ban on travel from West Africa. Obama said experts tell him that "a flat-out travel ban is not the way to go" because current screening measures at airports are working.

He said he had no philosophical objection to a travel ban but that some travellers might attempt to enter the United States by avoiding screening measures, which could lead to more Ebola cases, not less, Obama said.

Many House of Representatives members have joined calls for a ban on travel from the hardest-hit West African countries: Liberia, Sierra Leone and Guinea.

CDC chief fields question about Canada

One lawmaker even briefly questioned whether the northern border might need to be better secured. That improbable reference to the 49th parallel came from a Tennessee Republican, who during a House hearing asked whether America's land borders were safe from the deadly virus.

"Do we need to worry about having an unsecure southern and northern border?" Marsha Blackburn, the vice-chair of the House energy committee, asked.

"Is that a big part of this problem?"

No, said the witness in question, Dr. Tom Frieden, the head of the Centers for Disease Control, who spent three hours Thursday taking questions from politicians amid escalating concern about the outbreak.

Blackburn, it turns out, misunderstood Frieden's reference to "overland travel" — he was talking about West Africa, not North America. But the exchange illustrates the growing anxiety in the U.S., and the extent to which the virus is becoming a political football with congressional elections barely two weeks away.

The fear is being fuelled by the death of a tourist from Africa, the spread of the disease to two medical workers who treated him and, finally, an admission from the health-care system that it goofed in the initial response.

Some politicians are using concern about the virus to attract attention to their own causes. A few Democrats have tried blaming conservatives for cuts to the health system, and some Republicans have used the outbreak to bolster their case for a clampdown on the southern border.

That's despite the fact that no cases have been detected in Mexico, or in Canada.

Corrections

An earlier version of this story from The Canadian Press incorrectly stated Canada is contributing $5 million to the fight against Ebola. In fact, Canada's current contribution is $35 million.

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